1. For roasting the prawns. Make the basting oil for roasting the prawns first. Pour a couple of spoonfuls of grapeseed oil into a saucepan over medium heat, add the carrots, celery, chillies, eschalot, garlic and ginger to this and sauté until lightly coloured. Add the reserved prawn shells and turn up the heat. Stir the mixture in the saucepan constantly until the shells have turned a red colour. Now add the remainder of the oil. Adjust the heat down and let the mixture cook until the oil is clear and coloured red. The cooking process will probably take an hour. Strain out the solids and reserve.
2. Make the dressing. Mix the mustard powder with the rice vinegar. Let sit for half an hour or so to let the powdered mustard reconstitute. Whisk in the fish sauce and then gradually whisk in the oil to form a thick emulsion. Taste and adjust with vinegar and fish sauce as desired: the resulting dressing should be salty, sour and powerfully pungent from the mustard powder.
3. Make the mung starch sheets. Whisk the starch, water and salt together in a heat conducting metal bowl.
4. Set the bowl over a pan of simmering water and stir slowly with a wooden spoon or rubber spatula. The mixture will slowly thicken as it cooks and become translucent.
5. Once cooked, pour into a straight-sided metal container that can hold half a litre capacity. Let set at room temperature. (This can be used the following day but would have hardened so steam before proceeding with recipe.) The mung starch jelly will set in about an hour after which time they can be cut with a knife into sheets with a sharp wet knife.
6. Make the dressing. Whisk the ingredients together to make a loose dressing. The taste should be hot, salty and sour.
7. To serve. Mix the prawn and the mung starch sheets together, place on serving dish, spoon dressing over and arrange garnish over this. Served at room temperature.
Note: To make chilli oil, snip the stems off 50g of dried red chillies and halve them. Shake out and discard the seeds. Crush the chillies into smaller pieces in a food processor and put in a heatproof bowl. Heat 250ml of peanut oil to 140°C and pour over the chillies. They will sizzle and toast. Set aside overnight before use.